This application relates to a catalytic converter, and more particularly to such a converter for controlling or purifying unburned emissions from exhaust gases. The invention is particularly applicable for use in controlling unburned emissions from exhaust gases of fuel burning engines and will be particularly described with reference thereto. However, it will be appreciated that the invention has broader aspects and may be used for controlling unburned emissions from other sources.
Exhaust gases from such fuel burning engines usually include unburned emissions in the form of hydrocarbons and carbon monoxide, and catalytic converters employing an oxidizing catalyst have been proposed for oxidizing such emissions.
It is thus known that oxidation of such emissions in the converter produces considerable heat, and the oxidizing catalyst will melt or otherwise become ineffective if it is heated to an extremely high or excessive temperature.
If an engine is continuously properly tuned, it would be possible to accurately predict the average unburned emissions in the exhaust gases and a catalytic converter could therefore be designed without serious overheating problems. However, engines are or become often out of tune due to fouled plugs or incorrect spark gap setting, carburetor foulding or improper adjustment, ignition fouling or improper adjustment, and other well-known causes. When the engine is in fact out of tune, the amount of unburned emissions in the exhaust gases are significantly increased. Oxidation of significantly greater amounts of unburned emissions than calculated in the converter causes overheating and destruction of the catalytic converter, rendering it inoperative.
Secondary air is thus commonly fed to the converter for providing excess free oxygen in the exhaust gases for reaction with the unburned emissions, with such secondary air usually preheated by feeding it into the exhaust manifold or exhaust pipe ahead of the converter. Feeding cool secondary air directly into the converter is not a satisfactory solution since the efficiency of the converter requires high temperature operation. Secondary air is commonly fed into the exhaust gases by venturi action or by an air pump driven by the engine.
When all secondary air is fed to the system upstream of the converter, a significant increase in the amount of unburned emissions can result in greater oxidation at the leading portion of the catalyst than was intended, and at least the initial or entry portion of the catalyst destroyed. One way of minimizing this problem is suggested by U.S. Pat. Nos. 3,180,712 to Hamblin and 3,186,806 to Stiles. According to these disclosures, the secondary air is delivered in stages along the length of the catalyst bed. When excessive amounts of unburned emissions are present, only a limited amount would be oxidized in each stage of the converter so the leading portion of the catalyst does not overheat. However, these prior arrangements do not provide an efficient and inexpensive manner of pre-heating the secondary air or uniformly distributing it at each stage.
Catalytic converters of the type described have also included temperature sensing arrangements for automatically bypassing exhaust gases around the converter when the converter overheats. Such arrangements are relatively ineffective because the converter is thereby inoperative and could remain so if the operator of the engine did not correct the problem responsible for the excess of the unburned emissions.